Differentiate between alkenes and alkynes, cis isomers and trans isomers. Describe how cis- trans isomers are used for night vision.

Alkenes have a C=C double bond whereas alkynes have a c-c triple bond. Carbon atom in alkenes is sp2 hybridized and in alkynes it is sp hybridized. Alkenes are planar, alkynes are cylindrical in shape.

Cis trans isomerism is possible in alkenes. When two similar groups are placed at the same side of the plane, cis and when they are on the opposite sides, trans isomer results.

Cis trans isomerism helps us see. We have Rhodopsin molecules in our retina that contain a protein called opsin and a derivative of vitamin A called 11-cis-retinal. In the dark, 11-cis-retinal fits nicely into the folds of the surrounding opsin. When light hits the rhodopsin, the 11-cis-retinal becomes all-trans retinal and it no longer fits into the cavity of opsin. The opsin and the all-trans retinal separate. The change in rhodopsin structure is eventually transmitted to the nerve cells in the eye and thence to the brain. The cis-trans isomerism of retinal is thus an important part of the vision process. An enzyme later catalyzes the change of all-trans-retinal back to 11-cis-retinal so that it can once again bind opsin and wait for the next exposure to light.